Effect pigments, also known as pearlescent or nacreous pigments, are based on the use of a laminar substrate such as mica or glass flake which has been coated with a metal oxide layer. These pigments exhibit pearl-like luster as a result of reflection and refraction of light, and depending on the thickness of the metal oxide layer, they can also exhibit interference color effects.
Titanium dioxide-coated mica and iron oxide-coated mica effect pigments are the effect pigments which are encountered most often on a commercial basis. Pigments in which the metal oxide has been over-coated with another material are also well known in the art.
The commercially available effect pigments which contain only a single coating of a high refractive index material provide only two reflecting interfaces between materials. These two material interfaces (and reflections) are therefore solely responsible for the reflectivity achieved from the platelet surface. A substantial percentage of the incident light is thus transmitted through the platelet and while this is necessary to create the nacreous appearance of the pigment, it also diminishes other desirable properties of the effect pigments such as luster, chromaticity and hiding power. To counteract this consequence, the art has either mixed the effect pigments with other pigments or added additional layers of transparent and/or selectively absorbing materials onto the effect pigment.
Examples of prior art describing multi-coated effect pigments include JP 7-246366, WO 98/53011, WO 98/53012 and U.S. Pat. No. 4,434,010. All of such prior art requires that each coated layer possess an optical thickness equal to a whole number multiple of a one-quarter of the wave-length at which interference is expected. Such construction of the so-called quarter-wave stacks is a widely accepted and implemented condition in the thin-film industries. Because of this limitation, a unique layer thickness combination is essential in order to create each individual one of the interference colors of the visible spectrum. The base substrate is the only dimension common to all of the compositions displaying different interference colors.
It has now been discovered that the adherence to the quarter-wave stack approach is unnecessary and suitable products, even with substantial gains in luster, chromaticity and hiding power, can be achieved without observing that requirement. Further, numerous other advantages can be realized.
It is accordingly the object of this invention to provide a new multi-layer effect pigment, including having improved luster, chromaticity and/or hiding power relative to other effect pigments.